This special fish is used in the quest to increase your Fishing skill cap to 300: Nat Pagle, Angler Extreme. It is one of the few Soulbound fish and is also Unique.
This fish is found on the eastern shore of Swamp of Sorrows. Be certain the description on your minimap says "Misty Reed Strand" or you will not be able to catch the fish. The strand is populated with Murlocs around 40th level, so you may need to search for a safe spot to fish from or use 
[Water Walking].